27Background: High-frequency transcranial random noise stimulation (hf-tRNS) is a 28 neuromodulatory technique consisting of the application of alternating current at random intensities 29 and frequencies. hf-tRNS induces random neural activity in the system that may boost the 30 sensitivity of neurons to weak inputs. Stochastic resonance is a nonlinear phenomenon whereby the 31 addition of an optimal amount of noise results in performance enhancement, whereas further noise 32 increments impair signal detection or discrimination. 33Objective: The aim of the study was to assess whether modulatory effects of hf-tRNS rely on the 34 stochastic resonance phenomenon, and what is the specific neural mechanism producing stochastic 35 resonance. 36Method: Observers performed a two-interval forced choice motion direction discrimination task in 37 which they had to report whether two moving patches presented in two temporal intervals had the 38 same or different motion directions. hf-tRNS was administered at five intensity levels (0.5, 0.75, 39 1.0, 1.5, and 2.25 mA). 40 Results:The results showed a significant improvement in performance when hf-tRNS was applied 41 at 1.5 mA, representing the optimal level of external noise. However, stimulation intensity at 2.25 42 mA significantly impaired direction discrimination performance. An equivalent noise (EN) analysis, 43 used to assess how hf-tRNS modulates the mechanisms underlying global motion processing, 44 showed an increment in motion signal integration with the optimal current intensity, but reduced 45 motion signal integration at 2.25 mA. 46Conclusion: These results indicate that hf-tRNS-induced noise modulates neural signal-to-noise 47 ratio in a way that is compatible with the stochastic resonance phenomenon. 48 49 Keywords: global motion, high-frequency transcranial random noise stimulation, stochastic 50 resonance, internal noise, global sampling 51 52 53Transcranial random noise stimulation (tRNS) is a non-invasive electrical brain stimulation 54 technique characterized by alternating current delivered at random frequencies and intensities. This 55 technique can be applied at its full frequency spectrum between 0.1 Hz and 640 Hz, at the low-56 frequency range between 0.1 Hz and 100 Hz (lf-tRNS), or at the high frequency range (hf-tRNS), 57 between 101-640 Hz [1] . Early studies found that 10 mins of hf-tRNS applied over the primary 58 motor cortex (M1) induced an increment in cortical excitability with after-effects lasting up to 60 59 min [1][2][3][4]. In the last decade, several experimental procedures have been used to assess the effects 60 of tRNS on different cognitive and sensory abilities in order to understand its mechanisms [5]. For 61 example, it has been demonstrated that hf-tRNS improves behavioural performance on visual tasks 62 [6], attenuates visual motion adaptation [7], facilitates facial identity perception [8] and enhances 63 perceptual learning [9-14]. Moreover, five days of training with concomitant hf-tRNS over the 64 bilateral dorsola...
Perceptual learning has been shown to produce an improvement of visual acuity (VA) and contrast sensitivity (CS) both in subjects with amblyopia and refractive defects such as myopia or presbyopia. Transcranial random noise stimulation (tRNS) has proven to be efficacious in accelerating neural plasticity and boosting perceptual learning in healthy participants. In this study, we investigated whether a short behavioral training regime using a contrast detection task combined with online tRNS was as effective in improving visual functions in participants with mild myopia compared to a 2-month behavioral training regime without tRNS (Camilleri et al., 2014). After 2 weeks of perceptual training in combination with tRNS, participants showed an improvement of 0.15 LogMAR in uncorrected VA (UCVA) that was comparable with that obtained after 8 weeks of training with no tRNS, and an improvement in uncorrected CS (UCCS) at various spatial frequencies (whereas no UCCS improvement was seen after 8 weeks of training with no tRNS). On the other hand, a control group that trained for 2 weeks without stimulation did not show any significant UCVA or UCCS improvement. These results suggest that the combination of behavioral and neuromodulatory techniques can be fast and efficacious in improving sight in individuals with mild myopia.
hf-tRNS interacts with the output neurons tuned to directions near to the directional signal, incrementing the signal-to-noise ratio and the pooling of local motion cues and thus increasing the sensitivity for global moving stimuli.
A long-held view of the visual system is that form and motion are independently analysed. However, there is physiological and psychophysical evidence of early interaction in the processing of form and motion. In this study, we used a combination of Glass patterns (GPs) and repetitive Transcranial Magnetic Stimulation (rTMS) to investigate in human observers the neural mechanisms underlying form-motion integration. GPs consist of randomly distributed dot pairs (dipoles) that induce the percept of an oriented stimulus. GPs can be either static or dynamic. Dynamic GPs have both a form component (i.e., orientation) and a non-directional motion component along the orientation axis. GPs were presented in two temporal intervals and observers were asked to discriminate the temporal interval containing the most coherent GP. rTMS was delivered over early visual area (V1/V2) and over area V5/MT shortly after the presentation of the GP in each interval. The results showed that rTMS applied over early visual areas affected the perception of static GPs, but the stimulation of area V5/MT did not affect observers' performance. On the other hand, rTMS was delivered over either V1/V2 or V5/MT strongly impaired the perception of dynamic GPs. These results suggest that early visual areas seem to be involved in the processing of the spatial structure of GPs, and interfering with the extraction of the global spatial structure also affects the extraction of the motion component, possibly interfering with early form-motion integration. However, visual area V5/MT is likely to be involved only in the processing of the motion component of dynamic GPs. These results suggest that motion and form cues may interact as early as V1/V2.
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